1. Field of the Invention
The present invention pertains to an electrostatic chuck structure which provides for the flow of heat transfer fluids to the surface of an electrostatic chuck. The structure comprises a conductive underlayer which contains a gas flow channel and an overlying layer of dielectric material. The structure is useful in preventing the breakdown of a heat transfer fluid fed through the electrostatic chuck to its surface to cool the bottom surface of a work piece such as a silicon wafer which resides upon the electrostatic chuck. The structure is also useful in preventing the penetration of semiconductor processing plasma into the heat transfer fluid openings in the electrostatic chuck.
2. Brief Description of the Background Art
U.S. Pat. No. 5,350,479 to Collins et al. issued Sep. 27, 1994, and hereby incorporated by reference, describes an electrostatic chuck for holding an article to be processed in a plasma reaction chamber. The electrostatic chuck includes a metal pedestal coated with a layer of dielectric material which contains a cooling gas distribution system for passing and distributing a cooling gas between the upper surface of the electrostatic chuck and an article supported on that surface. The gas distribution system includes a plurality of intersecting grooves formed entirely in the upper surface of the electrostatic chuck, with small gas distribution holes through intersections of the grooves.
The lifetime of an electrostatic chuck is affected by the presence of the gas distribution holes used to facilitate the heat transfer gas. In particular, when the electrostatic chuck is subjected to high power rf fields and high density plasmas immediately above the work piece, it is possible to have breakdown of the cooling gas due to arcing or glow discharge. Further, since there is a line of sight path between the article (typically a semiconductor substrate) supported on the upper, dielectric surface of the electrostatic chuck and the underlying conductive layer (such as aluminum) which forms the pedestal of the electrostatic chuck, arcing can occur along this path despite the fact that the gas distribution holes may be sized to minimize discontinuities in the electric field which can lead to breakdown of the cooling gas passing through the holes. Arcing or glow discharge at the surface of the semiconductor substrate can result in loss of the substrate. Arcing or glow discharge within the gas distribution holes deteriorates the dielectric layer and underlying aluminum layer of the electrostatic chuck itself.
Collins et al. recommends that the aluminum layer beneath the dielectric layer be cut back (away) beneath the dielectric layer immediately adjacent the gas distribution hole to reduce the possibility of arcing across the line of sight path from the semiconductor substrate to the aluminum layer.
U.S. Pat. No. 5,315,473 to Collins et al., issued May 24, 1994, and hereby incorporated by reference, describes methods of improving the clamping force of the electrostatic chuck among other features. In particular the composition of the dielectric material and the thickness of the dielectric layer are among the critical factors. Generally, the thinner the dielectric layer, the greater the clamping force, all other factors held constant. However, there are practical limitations which limit the reduction of thickness of the dielectric layer. For dielectric layers approximately 1 mil or less in thickness, it has been found that the dielectric material breaks down and loses its insulating properties at voltages required to overcome air gaps between the article being processed and the underlying platform.
European Patent Application No. 93309608.3 of Collins et al., published Jun. 14, 1994, and hereby incorporated by reference, describes the construction of an electrostatic chuck of the kind disclosed in U.S. Pat. No. 5,350,497 referenced above. The electrostatic chuck fabrication includes bead blasting of the aluminum pedestal, followed by spraying (e.g. plasma-spraying of a dielectric material such as alumina or alumina/titania) upon the bead-blasted surface. Typically the sprayed thickness is greater than the final desired thickness, e.g. 15-20 mils (380-508 microns). After the dielectric material has been applied, it is ground back to a layer having a desired final thickness, for example, 7 mils (180 microns). The upper surface of the dielectric layer is then processed to provide a pattern of cooling gas distribution grooves over the surface of the layer and perforations through the dielectric layer which connect with cooling gas distribution cavities within the underlying aluminum pedestal. In some instances, the gas distribution cavities within the underlying aluminum pedestal are prepared in advance of application of the dielectric layer, and in other instances, the gas distribution cavities in the aluminum pedestal are prepared simultaneously with the perforations through the dielectric layer. Typically, the cooling gas distribution grooves are produced using a laser. The perforations through the dielectric layer are prepared by drilling using a mechanical drill or a laser. A preferred laser for drilling is an excimer UV laser (i.e. a short wave-length, high energy laser) run at a relatively low time averaged power level. This helps reduce the redepositing of drilled aluminum from the underlying thin layer onto the walls of the perforations and onto the surface of the dielectric. Presence of such aluminum can cause arcing across the dielectric layer. The perforations are frequently placed around the outer perimeter of the surface of the electrostatic chuck. For an 8 inch silicon wafer electrostatic chuck, there are about 180 such perforations which form a ring-like structure around the outer perimeter of the electrostatic chuck. Each perforation has a diameter which is approximately 0.007.+-.0.001 inch (0.175.+-.0.025 mm).
While micro-drilling through the composite dielectric layer overlaying the aluminum pedestal to provide the perforations described above provides a satisfactory gas passage, it fails to address the rf plasma environment that seeks the interface between the dielectric alumina coating and the aluminum substrate. Moreover, the laser drilling process ablates the aluminum beneath the dielectric layer as drilling proceeds and this ablate condenses or deposits in the bore, thus coating ceramic surfaces of the bore. Due to this mechanism, at least the lower portion of the hole may become a metallic conductor (aluminum) despite the use of a high aspect ratio (depth/diameter) for the gas passage. The removal of the machined micro chips slurry from the distribution hole is a difficult task, and is compounded by any migration of aluminum particles up through the dielectric gas distribution hole during drilling. Presence of machined micro chips slurry is a source of contaminant in the micro electronic environment.